Examples of blocking means for changing one or a plurality of electric wires from a conductive state to a blocked state when an overcurrent flows include a circuit breaker, a fuse, a residual current device, and a compound of a circuit breaker and a residual current device (Non-patent literature 1). FIG. 1 is a block diagram of a representative circuit breaker shown in a product catalog (electronic circuit breaker) of the present applicant. FIG. 2 is a diagram showing operating characteristics of a circuit breaker 900 shown in FIG. 1. The horizontal axis of FIG. 2 denotes current value (%) where the current rating of a power current at a commercial frequency is set to 100%, and the vertical axis denotes operating time (seconds, minutes). Thus, the circuit breaker does not operate when the current value is at the current rating 100%. Blocking is possible for several seconds to several minutes when the current value is, for example, twice the current rating (200%). In FIG. 1, the circuit breaker 900 is constituted by: electric wires 991, 992, and 993; current transformers (CT) 901, 902, and 903 that detect current flowing in the electric wires 991, 992, and 993; a rectifier circuit 910 that rectifies output current from the current transformers 901, 902, and 903; circuits 920 and 930 that output information of each operating condition; a switchgear 962 that is connected in series to the electric wires 991, 992, and 993 and that can disconnect the electric wires; a trip coil 961 that drives the switchgear 962; a trigger circuit 950 that applies a current to the trip coil 961 based on the information from the circuits 920 and 930; and the like. Several terminals are used to set the operating conditions, thereby various operating characteristics as shown in FIG. 2 can be obtained.
FIG. 3 shows a relationship between the current flowing through the electric wires and the output voltage of a current transformer when the load impedance is connected to the current transformer. The horizontal axis denotes the current flowing through the electric wires detected by the current transformer. When the current flowing through the electric wires is low, the effective value and the peak value of the output voltage of the current transformer are proportional to the current (linear region). Therefore, when the current is a commercial alternating current, the output from the current transformer is like a sine wave. However, as the current flowing through the electric wires becomes high, the effective value of the output voltage of the current transformer is not proportional to the current (nonlinear region). Even in the nonlinear region, it can be seen that the peak value becomes large as the current becomes high. Therefore, the nonlinear region does not result in a sine wave as shown in FIG. 3, even if the current flowing through the electric wires is a commercial alternating current. In case of the circuit breaker, the current transformers are designed to be operable in the linear region to ensure the operating characteristics as shown in FIG. 2. In other words, because current transformers with large core shapes are used to avoid magnetic saturation, the current transformers become large.
FIG. 4A shows an installation method of the circuit breaker. The circuit breaker is usually installed between the power side and the load side. The operating characteristics shown in FIG. 2 indicate operating characteristics required when the circuit breaker is installed as in FIG. 4A.
FIG. 4B shows an installation method of a fuse. The fuse is usually installed between the power side and the load side. An object of the circuit breaker and the fuse is to block the current of a commercial power supply when the current exceeds a desired value to prevent fire or the like.
FIG. 5 shows an example of an overvoltage protection device constituted by combining the circuit breaker with overvoltage protectors. An overvoltage protection device 2000 is constituted by overvoltage protectors 801, 802, and 803, the circuit breaker 900, and the like. There are various types of the overvoltage protectors 801, 802, and 803 which are generally constituted by discharge tubes, varistors, and the like. The overvoltage protectors 801, 802, and 803 are usually high impedance (state that the current does not flow). When a lightning surge voltage is applied between electric wires connecting the power side and the load side and the ground 890, the overvoltage protectors 801, 802, and 803 become low impedance and become substantially short circuited (state that the lightning surge current flows). Therefore, the lightning surge current is discharged to the ground 890. However, even if there is no more lightning surge current, the state of the low impedance of the overvoltage protectors may continue, and the commercial power supply may continue to flow into the ground 890. The current of the commercial power supply still flowing when there is no more lightning surge current will be called a follow current. The circuit breaker 900 of FIG. 5 is connected in series to the overvoltage protectors 801, 802, and 803 to block the follow current. An example of the method for blocking the follow current includes a method using fuses 821, 822, and 823 as shown in FIG. 6. Conventionally, as a method for blocking the follow current of the overvoltage protectors, a circuit breaker or fuses that primarily block the current when the current of the commercial power supply becomes excessive have been alternatively used.
When the circuit breaker or the fuses are alternatively used, the lightning surge current may cause malfunction and disconnection of the circuit breaker or the fuses, and lightning surge current may not be able to be released to the ground 890. Meanwhile, if a circuit breaker and fuses with high current rating are used to avoid the disconnected state under the lightning surge current, the follow current lower than the current rating cannot be blocked. Furthermore, the circuit breaker or the fuses become large. Although, there has been such a problem conventionally, conventional overvoltage protectors could only handle lightning surge peak current of up to about 10 kA. Therefore, the circuit breaker or the fuses could only handle lightning surge peak current of up to about 10 kA. There were many examples of alternatively using circuit breaker or fuses with current rating of about 20 A or 30 A that can withstand the lightning surge peak current. In that case too, there was a problem that the follow current below the current rating could not be blocked. However, the maximum value of the lightning surge current flowing in one electric wire is about 50 kA, and overvoltage protectors of 50 kA or more have been developed that are used where the lightning surge current flows. When alternatively using a circuit breaker or fuses that can withstand the lightning surge current of 50 kA and that can avoid disconnection, a circuit breaker or fuses with 100 A or more of current rating need to be selected. If the current rating is increased this way, blocking of a high follow current also becomes impossible. Problems caused by increasing the current rating have become apparent, such as the electric wires may burn out and a physically large circuit breaker or fuses are necessary.
Other than the circuit breaker or the fuses, there are residual current devices as means for blocking. FIG. 7 is a functional configuration example of a residual current device shown in Non-patent literature 1 (FIG. 6.5, p. 120). A residual current device 700 is constituted by a zero phase current transformer (ZCT) 701 that detects the vector sum of the currents flowing through a plurality of electric wires, an amplifier 710 that amplifies an output from the ZCT 701, a switchgear 762 connected to the electric wires in series and capable of disconnecting the electric wires, a trip coil 761 that drives the switchgear 762, a switching part 750 that applies a current to the trip coil 761 based on the output from the amplifier 710, and the like. When the follow current occurs in one overvoltage protector (in case of one-wire earth fault), the ZCT 701 of the residual current device 700 can detect even a low current. When the low impedance state occurs in two or more overvoltage protectors (in case of two-wire earth fault or three-wire earth fault), two or three electric wires are short circuited through the connection to the same ground 890 (interphase short circuit). In case of the interphase short circuit, the directions of the currents of two or three electric wires are opposite. In such a case, since the currents cancel each other, the ZCT 701 cannot detect the follow current. Therefore, the residual current device cannot be used as a disconnector for the overvoltage protection device.
Non-patent literature 1: “How to Select and Use Breakers and Switchgears” by Hiroichi Nakajima, Ohmsha, Chapter 6 How to Select and Use Low-Voltage Breakers, pp. 116-121.